I think this might work better if you slid a knex rod (probably blue) into the dart so that the ram has something solid to impart it's energy to, as the spongy dart would absorb much of the energy that should be propelling the dart forward.
I also think that getting the right sized barrel would be a challenge.

i think that if any way, you can have a mag of nerf darts and then you can have the ram hit the nerf darts, but there will be nothing in the way to stop the dart. either that, or put a rod through the hole in the back of the nerf bullet, and have the ram hit that. another way is to make the ram hit one bullet, and that bullet can travel farther down the barrel to reache the nerf bullet. hope this helps!
;)